In the field of graphic arts, an ultrahigh contrast photographic material is required for achieving satisfactory halftone dot reproduction of a continuous tone or reproduction of a line image in the process of making a lithographic printing plate. For many years these ultrahigh contrast photographic images were obtained by developing a `lith` emulsion (usually high in silver chloride content) in a hydroquinone, low sulphite, `lith` developer by the process known as infectious development. However, such low sulphite developers are inherently unstable and are particularly inappropriate for machine processing.
More recently an image formation system providing ultrahigh contrast where the gamma (contrast) exceeds 10 has been provided conventionally in a material wherein silver halide bearing a surface latent image is developed in the presence of a specific acylhydrazine (also known as a nucleating agent) which can be incorporated into the photographic material or into the developer. The pH of the developer solution is usually in the range 10.0 to 12.3, typically about 11.5, and the developer includes conventional amounts of sulphite, hydroquinone and possibly metol or a pyrazolidone. While such a process is better than the low sulphite `lith` process, the developer still has a high pH requirement for it to function correctly. Such a solution is not as stable as is desirable. Additionally, high pH solutions are environmentally undesirable because of the care needed in handling and disposing of the effluent.
Unfortunately, light sensitive materials whose contrast is enhanced by the presence of a hydrazine nucleating agent show large variations in their photographic properties as the developer is exhausted or through the course of time, for example as the pH of the developer varies and in particular as the pH is lowered. The pH of the developer can vary for a number of reasons: for example, exhaustion and absorption of carbon dioxide causes the pH to drop whilst air oxidation causes the pH to rise, as can concentration through evaporation.
It is also known that a developer solution having a pH below 11 can be employed by using certain hydrazides active at this pH. Hydrazides proposed for such use are described, for example, in U.S. Pat. Nos. 4,278,748; 4,031,127; 4,030,925 and 4,323,643 and in EP-A-0 333 435. A nucleator containing both a hydrazide moiety and a nicotinamide moiety is disclosed in U.S. Pat. No. 5,288,590.
Developer solutions with these low pHs can also be used by the introduction of a contrast-promoting agent (commonly called a booster) to give adequate activity. The booster can be incorporated into the photographic layer or may be dissolved in the developer solution. The booster may be, for example, one of the boosters as described in U.S. Pat. No. 5,316,889 or an amine booster as described in U.S. Pat. Nos. 4,269,929; 4,668,605, 4,740,452 or EP-A-0 364 166. Compounds bearing different functionalities e.g. phosphonium and pyridinium, have also been shown to be active, as described in U.S. Pat. No. 5,744,279.
The disadvantages connected with the necessity of using a booster to promote nucleation are numerous. Some materials are toxic, some are excessively volatile, some have unpleasant odours, some tend to form azeotropes with water, some build up in the developer during processing, some are insufficiently soluble in an aqueous alkaline photographic developing solution and some are costly, yet must be used at a relatively high concentration such that they contribute substantially to the overall cost of the material. Moreover, many boosters exhibit a degree of activity as contrast-promoters that is less than is desired for commercial operation. In addition, a photographic system depending on the combination of nucleator and booster is an exceedingly complex system which makes its performance particularly sensitive to variation. It would be desirable therefore if good nucleation could be achieved in the absence of such a booster or with a reduced amount of such a booster.
In the non-image areas on the processed film unwanted small dots can appear and this is called `pepper fog`. This is due to unintentionally fogged grains developing and being amplified by the nucleation process and being rendered visible. Nucleators which are unstable or more active and diffuse more rapidly can result in more and larger pepper fog spots. In high contrast materials therefore a balance needs to be achieved between vigorous development and pepper fog.
Another consideration is chemical spread (or image spread) which is a measure of the increase in size of developed dots or lines produced by nucleation of the edge of the image area causing development of the image boundary beyond the original exposed edge. This spread is small but measurable and can reduce the resolution of very fine lines.
The problem is therefore to provide a nucleator for incorporation into a photographic material or into the developer which gives ultrahigh contrast but which at the same time shows less sensitivity to variations in the developing solution, such as pH, provides sufficient activity in the presence of reduced amounts of a booster or ideally in the absence of booster, provides lower chemical spread and has significantly reduced pepper fog in the photographic material.
It has been found that these objectives can be achieved by the use of a nucleating agent comprising a dimeric molecule wherein the monomers, which are linked by a linking group, each comprise an acylhydrazide and a nicotinamide moiety. Such a nucleating agent can lead to unexpectedly good nucleation even in the absence of a booster and also in a developer whose pH is variable, with concomitant lower chemical spread and pepper fog.